


Old and New Habits

by Evrelai



Category: Undertale (Video Game)
Genre: Ambiguous Gender, Ambiguous Genitals, Double Penetration, Light Bondage, Multiple Orgasms, Other, Overstimulation, dubcon maybe??, he's in a Mood, i think, idk let's be safe, reader left ambiguous so anyone and everyone can enjoy yaaay, slightly feral/agressive gaster, voidy goopy gaster
Language: English
Status: Completed
Published: 2016-03-03
Updated: 2016-03-03
Packaged: 2018-05-24 12:00:21
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 1,885
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/6153001
Author URL: https://archiveofourown.org/users/Evrelai/pseuds/Evrelai
Summary: <blockquote class="userstuff">
              <p>In which Gaster gets a little freakier than usual. Based off a conversation or two with my friend, who spurred me to write this! Cross-posted from my Tumblr, I felt like this could find a home here too!</p>
            </blockquote>





	Old and New Habits

**Author's Note:**

> This is the first fic I've written in quite a while that ever saw the light of day lmao. Plus my first post here! Hopefully it's a good one!
> 
> I feel like this kinda fell apart a little at the end, but hopefully you like it anyway! Sorry it's kinda short. ; v;

The day itself had been unremarkable. It was typical by now, though–-you, upstairs, flipping through channels on the TV (there wasn’t a very impressive selection, admittedly) and scrolling your favorite websites on your phone, and the once-again royal scientist holed up in his lab downstairs. It wasn’t that he didn’t like to spend time with you, he just…was one to become very engrossed in his work. Very very much so. There were times you could swear he was addicted to it.

And it wasn’t as if you couldn’t go down and visit him. Despite his focus he still enjoyed conversation with you. It just felt rude, in your opinion, to bother him in his personal space, despite his multiple reassurances that it wasn’t. Tonight though…things felt a little strange. He’d seemed a little more aloof than usual earlier, and yourself…well. You couldn’t help but feel at least a little clingy now. He hadn’t been up all day…

From somewhere in the house a door opened, then creaked as it was shut again. A shifting noise followed it, all the way up the stairs. Ah…he was finally emerging from his little science cave.

You were about to say something when you felt a presence behind you, looming and almost heavier than what you were used to. Familiar hands ghosted across your shoulders before wrapping around you, followed soon by a face you knew all too well coming to rest against you, almost into the crook of your neck.

“Hello, Gaster.” You hummed softly, raising a hand to rest on his arm.

Or. Well…you tried, only to find your fingers squishing into him along with the rest of the parts of you that were touching him. Ah, he’d gone goopy–-a state he’d grown accustomed to while in the void. According to him it had been near-constant in the black abyss...but, now that he was corporeal again, he wasn’t bound to this state, but he still reverted back to it from time to time…primarily when he was upset.

Instantly, you were concerned.

“Is something wrong…?”

Gaster shifted, his demeanor throwing you off. It wasn’t the usual sad and pouty feeling you usually got. This time it was…aggressive, almost predatory. It would be off-putting, if it weren’t for the way his arms were laced about you.

“No.” His reply was clipped, muttered, almost hissed. You were about to question him further when some sort of noise bubbled from in his throat, vaguely threatening and almost…possessive.

Without warning, extra pairs of hands grasped and lifted you over the back of the couch and into his goopy waiting arms. He was quick to get moving, shifting mostly silently down the hallway. There was a strange look in his eyes, faintly glowing their respective blue and orange hues. Anything you could’ve thought to say was chased out of your mouth with the way he glanced down at you, face still and stoic as an old-fashioned photograph. And yet, paradoxically, his expression held all the possessiveness and mischief in the world.

Silently, still carrying you, he slipped into the bedroom, an extra hand pulling the door closed behind him with a decisive click.

Gaster was gentle in setting you down on the bed, as if you were fragile enough to break. He stared at you a moment, making the air around the two of you slightly uncomfortable. Surely by now it was obvious what the scientist had on his mind…but what exactly he was up to you couldn’t place a finger on. He made the same noise as before again, a rumble that sounded somewhere between a purr and a growl.

“Um…Gast–-”

You were cut off by tendrils of inky black goo descending upon you, twirling about your arms and legs and neck and pushing you back roughly into the mattress. He kept you still as an overwhelming amount of extra hands began to slide across your skin, caressing literally every inch of your body. Eager fingers pried at your clothing while odd-colored pinpricks observed from the foot of the bed, looming over your squirming form.

“You are mine…” He sighed aggressively, sounding both possessive and whimsical in the same breath. A few hands slid beneath your waistband, mercilessly roving over the most sensitive areas that lie just between your thighs. You let out a sigh of a moan, causing him to smirk. A few fingers began to rub and stroke while others still prodded at your entrance. Another moan escaped your lips, and a rough chuckle came from somewhere in his gut.

“You are precious…” he grumbled, yet another extraneous hand roaming up from your stomach all the way to your mouth, boney fingertips gracing across your lips. “Precious and mine.” The last word was hissed in such a way that sent a pang of heat straight to your middle, causing you to moan again.

He was hardly exerting himself and yet you were already melting in his grasp. The fingers at your entrance pushed their way inside and began to thrust, sending your head back further into the sheets.

When you tried to moan, the fingers that had been feeling the softness of your lips intruded their way into your mouth, pressing against your tongue and muffling the noise coming from your throat. You sighed and mumbled around them before a thought occurred to you. You pressed your tongue back up against Gaster’s digits, running it along the bottoms before giving them a little suckle, humming your approval at his ministrations as you went.

Your actions had a profound effect on him, earning you a more fervent hiss and more intensely glowing eyes. Instantly he withdrew all the hands within you and yanked you closer to him, another rumble leaving him as two new sources of orange and blue light illuminated the small space between your crotch and his body.

Two (two!) dicks, one of either color, had formed out of his body, solid and eager. You’d only ever seen the single black appendage in his normal, non-goopy form…this was definitely something new.

You barely had time to absorb the sight before more tendrils had come to feverishly remove your clothes, a scowl of impatience now on the face of your inky lover.

Gaster pressed forward, one of his two members sliding easily into you while the other rested atop your abdomen. His eyes lidded and flickered before the grip of tendrils binding you tightened and he began to thrust, desperate and vigorous. He went deep into you, managing to hit all the spots that drove you crazy. You weren’t used to so much so soon, but any qualms you could’ve had with the situation you were in quickly melted away.

The bed beneath you creaked in protest of Gaster’s force, though it was nearly drowned out by your moans of ecstasy. You squirmed against your bindings, toes curling into the sheets.

Soon enough Gaster shifted, his new angle allowing him to hit the most sensitive areas within you. Your moans only grew in volume and desperation, your whole body beginning to tremble in your delight.

“G–Gaster–-I–-hhh–-” Your words came as sighs, riding on the exhales of your whines and wails. “I–-”

Your words were silenced by another drippy tentacle pressing its way into your mouth while it was open. It slipped down your throat, enough to muffle you, but not so much as to choke you.

“Shhh…” Gaster chuckled, his thrusts becoming quicker as you whimpered around a mouthful of goo. It was becoming too much, and a warmth was blossoming in your gut…

Not too long after, you came, muscles clenching and an urgent groan leaving you in a wash of heat, speckles dancing behind your eyes. Gaster followed not too long after, filling you with a thick black goop. You fell limp beneath him, chest heaving.

The binds, however, did not loosen. Gaster shifted, and you hardly had enough time to give him a quizzical look before he began to thrust inside you again, albeit a bit slower this time. Instantly the heat returned to you, washing over your tired nerves.

Viscous, slippery goo slowly spread from Gaster to you, joining his member already within. Over time he added more and more, stretching you more than you ever thought you could be. His eyes were still hungry, raking across your body like he hadn’t seen you in years. He was far from finished with you, and for a second you wondered if you’d make it through the night without passing out.

Once satisfied with his work, Gaster removed the extra goo from you, only to replace it with his other member, the appendage throbbing impatiently as he pressed it into you. You felt impossibly full, whimpering softly around the tendril still in your mouth.

He afforded you only a few seconds to acclimate to the sensation before he was rocking into you again, almost more feverishly than before. It wasn’t long before the discomfort of being stretched melted away into mind-numbing pleasure, sending your already haggard nerves alight.

It was as if your body had caught fire in all the best ways, sending your back arching off the mattress and your eyes back in your skull. Your mouth was open in a silent scream, the pleasure leaving you numb and stealing your exclamations of enjoyment. When noise finally did come from you, it came in screams that you initially couldn’t even recognize as your own, strangled and warped by the goo slipping farther down your throat.

Almost too quickly another orgasm rocked your form, throwing your body into tremors. Gaster never stopped or slowed, powering through your cries. You could hardly feel your body anymore, your vision nearly going white behind your tightly-clenched eyelids. Every nerve felt raw, and small tears began to well at the corners of your eyes. You weren’t even sure if you could recall exactly where you were–-all you knew was world-ending pleasure and the slimy grip of W.D. Gaster’s tentacles.

Your last orgasm came nearly in tandem with his, sending stars exploding across your vision for what felt like the hundredth time that night. Gaster’s weight was heavy on you as he let out a groan that sounded nearly like a roar, pumping you so full with even more of his seed that it melted out of you, running out in thick streams when he finally moved to pull out from you.

His grip finally released you, though you were too spent to move or do much else than pant, your breaths coming in ragged gulps. Your body trembled and you mumbled out whimpers as the last few aftershocks rolled through you.

Slowly, Gaster lowered his weight onto the bed beside you before pulling you flush against him. Where his goopy body was normally clammy, it was warm and soft, and you didn’t even mind all that much that you sank into him a little.

His arms about you were just as possessive as before, though his voice was tired and gentler when he spoke up again.

“You are mine.” He repeated again, a soft kiss being pressed against your forehead.

“…I am.” You whispered back, falling asleep in his grip mere moments after.


End file.
